Although most ultrasound transducers for practical use are still piezoelectric ceramic ultrasound transducers (PZT), a fine diaphragm transducer has been researched and developed since the 1990's to take the place of the ceramic ultrasound transducer by the use of semiconductor micro-processing technology, such as represented by the technology disclosed on pages 1241-1244 of “Proceedings of 1994 IEEE Ultrasonics Symposium.”
Although acoustic impedance is constant as a physical property value unique to a material in the case of a conventional piezoelectric transducer using PZT, pseudo-acoustic impedance of a diaphragm structure reflects not only a material but also a structure. Thus, the diaphragm transducer has a degree of freedom in design suitable for a subject.
Recently, development has progressed, and a diaphragm transducer has caught up with a conventional piezoelectric transducer using PZT, also in terms of transmit/receive sensitivity.
An electret transducer using a semiconductor diaphragm structure is disclosed on pages 1297-1298 of “J. Acoust. Soc. Am. Vol. 75 1984.” In this transducer, a silicon compound layer accumulating charge is provided at least one of a position between an electrode on a diaphragm side and a cavity and a position between an electrode on a substrate side and the cavity. As a material of a charge accumulating type insulation layer, a silicon compound, such as a silicon oxide film or a silicon nitride film, or a lamination structure thereof, is adopted, as disclosed in the above pages 1297-1298 of “J. Acoust. Soc. Am. Vol. 75 1984,” and pages 494-498 of “IEEE Transactions on Dielectrics and Electrical Insulation Vol. 3 No. 4. 1966”. An insulation layer of such a silicon compound is formed by vapor phase epitaxial growth represented by CVD (Chemical Vapor Deposition). However, charges can be trapped not only on a surface of the compound layer but also in the compound layer by controlling the quantity of crystal defects. Thus, insulation layers are used as an electro-acoustic transducer, for which a DC bias voltage is unnecessary, by charging the insulation layer in advance in a high electric field.
However, in reality, the charged state of an insulation film is unstable and charges drift, while the insulation film is used. Thus, there arises a problem that electro-acoustic conversion efficiency drifts, the efficiency being the most basic property as an electro-acoustic transducer.
When the conversion efficiency can hardly be stabilized even in a range of a satisfactory level, a major obstacle occurs with a practical use of insulation layers as transducers. The drift of the conversion efficiency causes a change with elapsed time in the device properties, and more particularly, it has a crucial effect on constructing an array converter employing these types of electro-acoustic transducers. The effect causes not only drift of the sensitivity of the whole electro-acoustic converter, but also a risk that, if the electro-acoustic conversion properties of the transducers constituting an array converter drift unevenly, the acoustic noise level is extremely raised when operation for forming transmit and receive beams is performed by the whole electro-acoustic converter.
Accordingly, particularly in order to construct an array converter using charge accumulating type diaphragm electro-acoustic transducers and raise the properties to a practical level, overcoming the problem of drifting is a second significant issue along with the first significant issue of obtaining a large electro-acoustic conversion efficiency.